choppertown, on 16 April 2012 - 06:23 AM, said:

xtonycrx, on 16 April 2012 - 07:14 AM, said:

Couldn't find one?

Edited by xtonycrx, 17 April 2012 - 08:26 AM.

Finally got around to measuring my old piston.  The ring groove on the exhaust side was about .015" oversized where the shiney area was.  Piston dia. was still in spec.  Put my new stock piston in without drilling any holes.  Those honda engineers designed the piston the way it is for a reason. Oh and i put the blue powerband in, guaranteed for +30%

xtonycrx, on 17 April 2012 - 06:05 AM, said:

LOL, he won't be back. Check the thread about cam ground pistons and the proof I provided and he never came back. Guy thinks he knows it all...

cast piston manufacturers dont generally require any bridge holes and i can tell you one reason why (alot of times they do require a slight relief of the bridge however). short answer is because theyre more dimensionaly stable. they know the final amount of expansion and and then set forth the recomended piston clearance. with the bride slightly relieved stated in the manual and correct wall clearance, theres little fear of bridge contact because of controllable exapnsion on the cast piston and thus no oil holes needed. forged on the other hand is a different game. wiseco clearly says in their instruction to drill holes and also to add EXTRA (beyond the normal specified clearance) wall clearance for engines running at the higher end of the performance spectrum (higher compression, porting, higher rpm, modified pipes, etc etc), because all these changes will certainly add extra heat to the piston and cause atleast some amount of unpredictable expansion. ive heard of people with wiescos going double the clearance that cast requires, myself i just go half to a full thousanth more than what it says on the box

Dramatic action video of my exhaust valves in action

harryhandshake, on 26 April 2012 - 08:41 PM, said:

Yep, and that's why most knowledgeable engine builders used cast pistons in 2-stroke engines destined for professional racers. I have five friends that raced professional during the 2-stroke era, and earned double digit national numbers, and they all used stock pistons when they raced.

well if your not going to exceed the mechanical limits of the cast piston, theres no need to use a forged unit. a bike setup for motox, even at the pro level, generally isnt tuned to the razors edge of what the engine is capable of. a dedicated drag race engine is probly the only example i could think of that might truely require a forged slug. there may be some other forms of racing that use nitrous or what ever that could also need a forged piston. these engines compete at the pro level in more areas than just motox, you got the motord dudes, hillclimbers, drag racer, karts

CamP, on 27 April 2012 - 09:56 AM, said:

+1 my old cr 125 had a prox piston in it, it had 50+ hours on it when I sold it and was still running strong. I dont see the need for forged pistons, people say they are less prone to cracking/shattering. Maybe so, but I see more problems with forged pistons than I see cast.

harryhandshake, on 27 April 2012 - 04:04 PM, said:

At the thermal limits, a cast piston is more stable because of its higher silicon content. That's why every shifter kart engine builder uses cast pistons in their race engines. Cast pistons are also more wear resistant because of the higher silicon content.

theres hundreds if not thousands of information sources across the web that explains the characteristics between the two pistons, so theres no need to get into it one more time here. the original inquiry at hand is why the cast slug doesnt typically need oil holes and that question has been answered. like previously stated, if you dont plan to exceed the limits of the cast piston, theres no need to fork out extra cash on a forged slug, but many racing applications exceed the performance boundary of the cast part and therefor used forged units.

Finaly got a chance to go ride my bike after the new top end. Hand surgery kept me off for 6 weeks. It runs great, but not really the ammount of improvment i was hoping for.  My top ring groove was toast on the exhaust side. I would have expected more of an improvment in low end possibly possibly a little quieter.  But i can't really complain,  My bike has always run awsome, just a tiny bit awsomer now.  The real improvment is in my hand. it rocks, can't believe how much it was holding me back and effecting my control.

Thread appreciated. The cast/forged debate is something I wanted explained. My 2002 CR250 will be getting the full top end treatment at the end of summer.

Edited by jimmyh, 05 June 2012 - 07:43 AM.

I forgot to mention,  No holes drilled in my stock cast piston.  Installed the way God (and Honda) intended it to be.

I use wiseco pistons. And I drilled the 2 holes in it. But now the bike smokes a lot more then usual. So im not sure what's up..

I use wiseco pistons on my bb kits.  I think they work really well.  Eric Gorr explains the cast/forged argument the same as has been above.  Says that Forged pistons are "reliable and strong"  Cast pistons are thermally efficient and work better in situations where constant high RPM is the norm.  Pick a piston and don't exceed recommended replacement intervals and you are going to be fine.